Tuesday 11 June 2013 15.01 EDT
First published on Tuesday 11 June 2013 15.01 EDT

The use of some of the most potent antibiotics available has surged among British farmers in the last decade, stoking fears that the burgeoning number of factory farms could greatly increase the risk of antibiotic-resistant strains of bacteria escaping and infecting people.

Medicines to treat meningitis, pneumonia and other serious diseases, called third and fourth generation cephalosporins, increased threefold from 2003-11, while drugs called fluoroquinolones – used for tuberculosis, C difficile, HIV-related infections and others – are up by 50% in the same period.

These antibiotics have been flagged by the World Health Organisation as some of the most important in human medicine, because they are our last line of defence against serious diseases, and are under threat from bacterial strains that are growing more resistant to them.

Their rapidly rising use on farms is a threat to people, experts told the Guardian. Christopher Thomas, professor of molecular genetics at Birmingham University, said: "There a lot of worry about whether we should be using the same antibiotics on a farm as we do in [human] clinics, as the resistance developed on farms could spread to humans. However good your hygiene [on farms], it is inevitable that resistant bacteria bred on the farm will get to humans."

The government does not track the use of veterinary antibiotics in detail – unlike their human counterparts – so it is impossible to tell how many animals are being treated, for what diseases, and whether the medicines are being used as a prophylactic or to treat diseases already present. The only data available is the total annual tonnage of antibiotics sold for agriculture, some of which could be left unused, and the dosage of which could vary widely.

Green campaigners are also unhappy that vets are allowed to profit by selling farmers the antibiotics they prescribe, a practice banned in many other countries because of conflicts of interest. The government confirmed there were no plans to change this practice.

Thomas said that despite guidelines on avoiding routine use of antibiotics, it was difficult to distinguish in practice. "There is a fine line where you have lots of animals together. For instance, in an intensive chicken rearing facility if you get one or two animals that get an infection, it's quite common for vets to decide they need to treat all the chickens in the facility just in case it has already spread to others that are not noticeably sick."

He said there were many ways in which antibiotic-resistant bacteria on a farm could spread to humans: by people working there, who could spread it to their families; by run-off water from the farm, and by meat from slaughtered animals getting into the food chain.

But Peter Borriello, chief executive of the Veterinary Medicines Directorate, the government agency that oversees antibiotic use on farms, said people should not be over-concerned as the UK farming industry was well-regulated and used fewer antibiotics than other EU countries, being about the 8th biggest user of farming antibiotics, but only the 14th biggest user of the potent fluoroquinolones.

"We are not aware of any major resistance problems and not aware of any major changes in veterinary pathogen populations with respect to their resistance potential," he said.

Another key issue is the import of meat and fish from countries where farming antibiotic use is higher than in the UK, such as the US, where 80% of antibiotic use is for animals.

A Defra spokesman said UK food importers were only allowed to buy from countries that met standards similar to those in operation in Europe. However, the use of antibiotics in other countries does not fall under these rules, so imported meat can come from much looser antibiotic regimes. Defra said there were checks in place to ensure infected meat did not enter the UK food chain.

There are also moves in the US to cut antibiotic use. Citing a recent study, Louise Slaughter, the only microbiologist in Congress, said: "This study ends any debate. The extreme overuse of antibiotics in livestock is endangering human health. For decades, the United States Food and Drug Administration has failed to act in the face of a growing threat. These findings make it clearer than ever that their failure is endangering human life. Starting today, the FDA must take strong federal action to reduce antibiotic use in livestock and protect human health."

Antibiotics are routinely used in fish farms, particularly in Latin America. David Santillo, a Greenpeace fellow at Exeter University, said: "Eating raw fish as sushi can raise additional concerns regarding food safety. Although properly prepared, stored and handled sushi should be free from harmful pathogens, the potential presence of antibiotic residues or even of antibiotic-resistant strains of bacteria remains a concern, especially for fish and shellfish farmed in parts of the world in which controls on the use of such chemically intensive disease treatments are lacking or harder to police and verify."

The Food Standards Agency said that if meat and fish were cooked properly, any bacteria living on them should be killed, and advised people to follow best practice in handling uncooked food. Avoiding sushi and steak tartare might be one way to cut risk, but Thomas said the government should be more active in testing for microbes in the food chain.

For Sue Weston, in Foston, Derbyshire, the question of antibiotic-resistant bacteria breeding and evolving on British factory farms and finding its way into the human population is not an abstract one of academic studies and best hygiene practice. She lives "a car's width" from the site of a proposed intensive 25,000-pig farming facility. Her 19-year-old son suffers a serious heart condition that makes him vulnerable to infections.

"I'm petrified," she said. "It beggars belief that they can build something like this so close to people when so many of the dangers are still not fully known."